Coal tar is a primary by-product material produced during the destructive distillation or carbonization of coal into coke. While the coke product is utilized as a fuel and reagent source in the steel industry, the coal tar material is distilled into a series of fractions, each of which are commercially viable products in their own right. A significant portion of the distilled coal tar material is the pitch residue. This material is utilized in the production of anodes for aluminum smelting, as well as electrodes for electric arc furnaces used in the steel industry. In evaluating the qualitative characteristics of the pitch material, the prior art has been primarily focused on the ability of the coal tar pitch material to provide a suitable binder used in the anode and electrode production processes. Various characteristics such as softening point, specific gravity, percentage of material insoluble in quinoline, also known as QI, and coking value have all served to characterize coal tar pitches for applicability in these various manufacturing processes and industries.
Softening point is the basic measurement utilized to determine the distillation process end point in coal tar pitch production and to establish the mixing, forming or impregnating temperatures in carbon product production. All softening points referred to herein are taken according to the Mettler method or ASTM Standard D3104. Additional characteristics described herein include QI, which is utilized to determine the quantity of solid and high molecular weight material in the pitch. QI may also be referred to as α-resin and the standard test methodology used to determine the QI as a weight percentage include either ASTM Standard D4746 or ASTM Standard D2318. Percentage of material insoluble in toluene, or TI, will also be referred to herein, and is determined through ASTM Standard D4072 or D4312.
Mirtchi and Noel, in a paper presented at Carbon '94 at Granada, Spain, entitled “Polycyclic Aromatic Hydrocarbons in Pitches Used in the Aluminum Industry,” described and categorized the PAH content of coal tar pitches. These materials were classified according to their carcinogenic or mutagenic effect on living organisms. The paper identified 14 PAH materials which are considered by the United States Environmental Protection Agency to be potentially harmful to public health. Each of the 14 materials is assigned a relative ranking of carcinogenic potency which is based on a standard arbitrary assignment of a factor of 1 to benzo(a)pyrene or B(a)P. Estimations of potential toxicity of a pitch material may be made by converting its total PAH content into a B(a)P equivalent which eliminates the necessity of referring to each of the 14 materials individually, providing a useful shorthand for the evaluation of a material's toxicity.
A typical coal tar binder pitch is characterized as shown in Table I.
TABLE ISoftening Point, ° C.111.3Toluene Insolubles, wt. %28.1Quinoline Insolubles, wt. %11.9Coking Value, Modified Conradson, wt. %55.7Ash, wt. %0.21Specific Gravity, 25/25° C.1.33Sulfur, wt. %0.6B(a)P Equivalent, ppm35,000
Two shortcomings with respect to the use of coal tar pitch in general, and more specifically in the aluminum industry, have recently emerged. The first is a heightened sensitivity to the environmental impact of this material and its utilization in aluminum smelting anodes. The other is a declining supply of crude coal tar from the coke-making process. Significant reductions in coke consumption, based upon a variety of factors, has reduced the availability of crude coal tar. This reduction in production of these raw materials is expected to escalate in the near future and alternative sources and substitute products have been sought for some period. No commercially attractive substitute for coal tar pitch in the aluminum industry has been developed, however.
There are two traditional methods of distilling coal tar, continuous and batch. Continuous distillation involves a constant feeding of the material to be distilled, i.e., coal tar, and the constant removal of the product or residue, i.e., coal tar pitch. Traditional continuous distillations are typically performed at pressures of between 60 mmHg and 120 mmHg and at temperatures of between 350° C. and 400° C. and are typically able to produce a coal tar pitch having a maximum softening point of approximately 140° C. Batch distillation can be thought of as taking place in a crucible, much like boiling water. High heat levels are developed as a result of the longer residence time of the coal tar in the crucible. Although higher softening points of up to 170° C. can be reached using batch distillation, the combination of high heat and longer residence time can often lead to decomposition of the coal tar pitch and the formation of unwanted mesophase pitch. Processing times for the distillation of coal tar using known continuous and batch distillation range from several minutes to several hours depending upon the coal tar pitch product to be produced.
High efficiency evaporative distillation processes are known that subject a material to elevated temperatures, generally in the range of 300° C. to 450° C., and reduced pressures generally in the range of 5 Torr or less, in a distillation vessel to evolve lower molecular weight, more volatile components from higher molecular weight, less volatile components. Such high efficiency evaporative distillation processes may be carried out using conventional distillation equipment having enhanced vacuum capabilities for operating at the above specified temperature and pressure ranges. In addition, high efficiency evaporative distillation processes may be carried out in an apparatus known as a wiped film evaporator, or WFE, and thus such processes are commonly referred, to as WFE processes. Similarly, high efficiency evaporative distillation processes may be carried out in an apparatus known as a thin film evaporator, and thus such processes are commonly referred to as thin film evaporator processes. WFE and thin film evaporator processes are often used as efficient, relatively quick ways to continuously distill a material. Generally, WFE and thin film evaporator processes involve forming a thin layer of a material on a heated surface, typically the interior wall of a vessel or chamber, generally in the range of 300° C. to 450° C., while simultaneously providing a reduced pressure, generally in the range of 5 Torr or less. In a WFE process, the thin layer of material is formed by a rotor in close proximity with the interior wall of the vessel. In contrast, in a thin film evaporator process, the thin film evaporator typically has a spinner configuration such that the thin layer of material is formed on the interior wall of the vessel as a result of centrifugal force. WFE and thin film evaporator processes are continuous processes as they involve the continuous ingress of feed material and egress of output material. Both wiped film evaporators and thin film evaporators are well known in the prior art.
One prior art WFE apparatus is described in Baird, U.S. Pat. No. 4,093,479. The apparatus described in Baird includes a cylindrical processing chamber or vessel. The processing chamber is surrounded by a temperature control jacket adapted to introduce a heat exchange fluid. The processing chamber includes a feed inlet at one end and a product outlet at the opposite end.
The processing chamber of the apparatus described in Baird also includes a vapor chamber having a vapor outlet. A condenser and a vacuum means may be placed in communication with the vapor outlet to permit condensation of the generated vapor under sub-atmospheric conditions. Extending from one end of the processing chamber to the other end is a tube-like motor-driven rotor. Extending axially outward from the rotor shaft are a plurality of radial rotor blades which are non-symmetrically twisted to extend radially from one end of the chamber to the other between the feed inlet and the product outlet. The rotor blades extend into a small but generally uniform closely spaced thin-film relationship with respect to the interior wall of the processing chamber so that, when the rotor rotates, the rotor blades provide a thin, wiped or turbulent film of the processing material on the interior wall of the processing chamber.
In operation, a material to be processed is introduced into the feed inlet by a pump or by gravity. The material is permitted to move downwardly and is formed into a thin-film on the interior wall of the processing, chamber by the rotating rotor blades. A heat-exchange fluid, such as steam, is introduced into the temperature control jacket so that the interior wall of the processing chamber is heated to a steady, pre-selected temperature to effect the controlled evaporation of the relatively volatile component of the processing material. A relatively non-volatile material is withdrawn from the product outlet, and the vaporized volatile material is withdrawn from the vapor chamber through the vapor outlet.
One of the major uses of coal tar pitch is as a binder for carbon/graphite products. These products range from anodes for the production of aluminum to fine grain graphite products for use in electric discharge machining. Carbon/graphite products contain two major components petroleum coke and coal tar pitch. Coal tar pitch is the binder that holds the structure together. One of ordinary skill in the art would know that coal tar pitch which has not been cross-linked is inherently graphitizable. The major steps in production of the finished product are mixing, forming, carbonization for carbon products, and carbonization followed by graphitization for graphite products. The major problem experienced with pitch in the process is evolution of volatiles during the carbonization step. Volatiles evolution causes two major problems: 1) emissions of organic compounds, and 2) reduction of the density of the finished baked product. Volatiles emissions are an environmental concern which must be addressed by either capture or destruction of the organic compounds generated. The reduction of the density of the carbon/graphite product results in an inferior product with reduced strength, increased reactivity, and increased electrical resistivity. An advantage therefore exists for carbon/graphite products having low yield of volatiles.
Automobile brakes are produced by binding a number of inorganic and organic substituents with phenolic resin. The process is in certain respects similar to the one discussed for the production of carbon/graphite products above. One of the major problems experienced with automobile brakes is a characteristic called fade. Fade is a reduction of the friction characteristics of the friction material when it becomes hot. Everyone who drives an automobile has experienced fade when the brake is being applied on a downhill grade. As the brake begins to get hot, the driver must push harder on the brake pedal to achieve the same braking capacity. It is believed that fade is caused by the heat instability of the phenolic resin binder of the friction material. As the brake gets hot the phenolic resin begins to decompose resulting in production of a gas layer between the two sliding components. This gas layer causes a loss of friction resulting in the need to push harder on the brake pedal. An advantage therefore exists for brake formulations resulting in a reduction of fade.
Aircraft brakes are produced by carbon impregnation of a carbon fiber preform. The process used for carbon impregnation is called chemical vapor infiltration. Chemical vapor infiltration is performed by coking methane gas in the preform to result in a carbon filled carbon fiber preform. The chemical vapor infiltration process is very time consuming with about 600 hours of processing time required to produce a finished product. An advantage therefore exists for a carbon infiltration process having a reduced time.
Natural rubber is used to produce many of the products we use each day. One rubber product which plays a great part in each of our lives is tires. A tire is produced from a number of different rubber formulations. Different formulations are used to produce the tread, sidewalls, belt coating, and rim. One of the most important characteristics of the different rubber formulations used to produce a tire is the adhesive properties for each of the rubber formulations for each other. An advantage therefore exists for a rubber formulation having increased adhesive properties.
Mesophase pitch is a highly structured pitch which is used in applications where strength or the ability to conduct heat or electricity is important. Significant work has been performed to produce mesophase pitch from coal tar pitch with limited success because of the quinoline insolubles content of the pitch. It has been shown that the quinoline insolubles particles in coal tar pitch hinder coalescence of the mesophase spheres causing a poor quality mesophase to be formed. Known methods of producing mesophase from coal tar pitch involve a filtration or centrifugation step for removing the quinoline insolubles. While these processes work quite well and allow for production of a high quality mesophase, they result in a very high cost of the mesophase product. An advantage therefore exists for a lower cost production of a high quality mesophase.